Interaction between UV and optical or IR lines

Interaction between UV and optical or IR lines - III...

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Interaction between UV and optical or IR lines . Several cases are known of line resonance, in which some emission line is stronger than can be produced by continuum processes. The prime example is the O III resonance-fluorescence mechanism found by Bowen. There is a very close coincidence between He II Lyman α at 303.78 Å and a pair of O III lines at 303.62, 303.80 Å. The O III ion is pumped into the upper state by absorption of He II line radiation (depending on local optical depth and velocity field), cascading back to the ground state via a set of about 20 lines in the near UV and optical. These have been seen in a number of Seyferts (e.g. Malkan 1986 ApJ 310, 679), providing some measure of how efficiently He II radiation is converted to O
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Unformatted text preview: III emission (and thus of a combination of optical depth and velocity shear). Finally, recall the use of emission lines as kinematic tracers . Thermal motions produce minimal linewidths for a given ion, with a distribution function of line-of-sight velocity This is small for atoms, and smallest for heavier ones. At 10 4 K, the 1/e width is 12 km/s for hydrogen and only 4.5 km/s for oxygen. Thus all the structure and widths seen in AGN spectra are from mass motions or some scattering process that can smear frequencies very strongly (a role for electron scattering has been found in a few cases)....
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This note was uploaded on 11/10/2011 for the course AST AST1002 taught by Professor Emilyhoward during the Fall '10 term at Broward College.

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